1. Field of the Invention
The present invention relates to a speaker and, more particularly, to an induction type speaker.
2. Description of the Prior Art
In a conventional dynamic type speaker, by allowing an audio signal current to flow through a voice coil in a DC magnetic field, a driving force is obtained. The audio signal current is ordinarily supplied from the outside to the voice coil through lead wires fixed to a paper cone diaphragm.
However, a conventional dynamic type speaker has lead wires, which presents a drawback in that the lead wires can be easily cut due to elastic fatigue or the like caused by the reciprocating motion of the diaphragm. Even in the case where the lead wires are not cut, there is a drawback in that since the linearity in the reciprocating motion of the diaphragm is obstructed by the spring force of the lead wires, a sound distortion can easily occur and the lead wires themselves resonate producing an abnormal sound. Furthermore, since the lead wires must be led out from a narrow gap of the speaker and must be positioned, adhered, and fixed properly, the assembly is troublesome.
An induction type speaker from which lead wires are eliminated and a driving coil is arranged near a voice coil wound around a voice coil bobbin has been disclosed in the Official Gazette of Japanese Patent Application Publication No. 27039/1981. An audio signal current is supplied to the driving coil and the audio signal is supplied from the driving coil to the voice coil by magnetic induction. That is, when an AC signal flows from an electric power amplifier with an audio frequency to the driving coil, an AC magnetic flux corresponding to the input waveform is generated. The AC magnetic flux closely interlinks the voice coil located at a very close distance. Since the voice coil itself is short-circuited, a short-circuit current flows through the voice coil by the AC magnetic flux.
Since the voice coil is located in the magnetic field which is produced by a pole piece and the peripheral magnetic poles, a force which is proportional to the product of the intensity of the magnetic field and the short-circuit current acts on the voice coil. The force is transferred from the voice coil to the voice coil bobbin and vibrates a cone-shaped diaphragm thereby generating sound from the diaphragm as in the ordinary speaker.
Although various drawbacks due to lead wires are eliminated in the above disclosed technique, the following other problems occur.
Since the voice coil is generally fixed to the voice coil bobbin by an adhesive agent, it is difficult transfer the driving force generated in the voice coil to the diaphragm.
In addition, there is a drawback in that it is difficult to satisfactorily radiate the heat generated by the short-circuit current in the voice coil.
In order to improve the sensitivity of the speaker, it is required to narrow the magnetic gap between the coil bobbin and the driving coil, and to wind the voice coil a number of times in the gap. Therefore, the diameter of metal wire used for the voice coil becomes smaller with a corresponding decrease in the heat capacity of the metal wire. Thus, in addition to the problems of the heat radiation as mentioned above, there is the disadvantage that the heat generation could cause a break in the smaller diameter voice coil wire thereby limiting the current capacity.
Further, there is a whereby continuous repeated exposure to heat causes the paper voice coil bobbin to carbonize.
Therefore, an induction type speaker from which a voice coil is eliminated has been proposed in the Official Gazette of Japanese Utility Model Registration Application Laid-open No. 105438/1975.
In such an induction type speaker 1, as shown in FIG. 1, a diaphragm 4, having an annular conductive portion 3, is supported in an annular magnetic gap portion 2 by a damper 10 so as to freely vibrate. A current feeding coil 5, which is mechanically separated from the diaphragm 4 and electrically coupled with the conductive portion 3 by the mutual inductive operation, is arranged on the side of a magnetic circuit.
The magnetic gap portion 2 is formed annularly between a top plate 7, which sandwiches a magnet 6 such as a ferrite or similar magnet and which constitutes a magnetic circuit, and a center pole 9 of a yoke plate 8. Damper 10, which supports the diaphragm 4 so as to freely vibrate, is arranged on top plate 7.
The diaphragm 4 has a dome shape and annular conductive portion 3 in its opening edge portion. The entire diaphragm 4 is a thin plate-shaped good conductor, constructed from aluminum, beryllium, magnesium, or the like. Further, as mentioned above, since the current feeding coil 5 is to be mechanically separated from the diaphragm 4 and electrically coupled with the annular conductive portion 3 by the mutual inductive operation, the current feeding coil 5 is arranged so as to face the annular conductive portion 3 in either a position of an outer or inner periphery or an opening edge portion of the conductive portion 3. In this case, the current feeding coil 5 is fixed to an outer periphery of the edge portion of the center pole 9. The speaker 1 constructed as mentioned above operates as follows:
First, when an AC signal corresponding to an audio signal or the like is supplied to the current feeding coil 5, an induction current of the same frequency is induced in the annular conductive portion 3 of the diaphragm 4 by the mutual inductive phenomenon due to the interlinked magnetic flux generated by the current feeding coil 5. The induction current in the conductive portion 3 acts on the DC magnetic field from the magnetic circuit of magnetic gap portion 2 so as to drive diaphragm 4 and generated a sound wave.
In the above disclosed technique proposed in the Official Gazette of Japanese Utility Model Registration Application Laid-open No. 105438/1975, the lead wires and voice coil are eliminated, so that the various drawbacks due to lead wires, voice coil, and the like are eliminated.
However, the above diaphragm must be ordinarily formed from a metal since it is necessary to generate the induction current in the conductive portion part of the diaphragm.
When the diaphragm is formed from a metal, it becomes heavy, so that the response sensitivity of the speaker is reduced.
In addition to reduction of the response sensitivity, with the diaphragm made of metal the mechanical loss is small and the diaphragm is relatively heavy. This results in a frequency characteristic of the speaker which is not flat, sharp resonance peaks appear as shown in FIG. 2. Further, upon reading resonance, it is difficult to brake or damp the diaphragm.
Therefore upon resonance, the sound quality deteriorates.
Furthermore, since the conductive portion of the diaphragm and the portions other than the conductive portion in the diaphragm are not insulated at all, there is a problem such that the induction current induced in the conductive portion leaks out of the conductive portion becoming a leakage current thereby reducing the induction current by the amount of the leakage current. The leakage current also reduces the force to drive the diaphragm, so that the response sensitivity of the speaker deteriorates.
In an induction type speaker, a high-pass filter is equivalently constructed on the input side. Therefore, limitation in the reproducing of low frequency sounds is presented such that low frequencies cannot be reproduced.
The diaphragm 4 reciprocates in the directions indicated by arrows U-D in FIG. 3 in accordance with the induction current.
In FIG. 3, assuming that a range of the DC magnetic field having a uniform magnetic flux distribution (hereafter, the uniform magnetic field range) is set to L1 and a length of conductive portion 3 is set to L2 wherein, the uniform magnetic field range L1 and length L2 are substantially equalized.
Now, consider the case where the diaphragm 4 moves by only a length l in the direction indicated by the arrow U so that, an edge portion 3a of the conductive portion 3 reaches a point P1 in the uniform magnetiac field range L1. In such a case, in only the portion of conductive portion 3 corresponding to length (L1-l) lies within the uniform magnetic field range L1. The other portions [that is, the length corresponding to L2-(L1-l)] of the conductive portion 3 move outside of the uniform magnetic field range L1.
When the conductive portion 3 is out of the uniform magnetic field range L1, the magnetic flux density greatly decreases and if the induction current remains constant, the driving force to the diaphragm 4 also greatly decreases. That is, the amplitude of the diaphragm 4 response increases in accordance with the induction current. When the conductive portion 3 is deviated greatly from inside the uniform magnetic field range L1, the driving force is reduced whereby, the amplitude of diaphragm 4 does not accurately respond to a change in audio vibration, so that the linearity is lost and a distortion occurs.